1. Field of the Invention
The present invention relates to a radiation measurement system, such as scintillation camera and chemical meter device, and more particularly to an apparatus for controlling the gain of the radiation measurement system.
2. Description of the Related Art
When radioactive radiation emitted from the inside of the human body enters into a scintillator, scintillation light is produced. This scintillation light is converted into an electric signal and is amplified by a predetermined gain by a photomultiplier tube (PMT). This electric signal is further amplified by a predetermined hain by a preamplifier, or the like, and outputted as an output signal. Thereafter, information such as a radiosotope (RI) distribution relating to the light emitting position in the scintillator is obtained on the basis of the output signal, so that the shape, position, size, and the like of the desired porotion of the human body can be obtained.
It sometimes happens during the measurement process that the properties of the change over time and that the gain of the PMT therefore becomes low. In order to prevent this, the gain of the preamplifier is corrected and the level of output signal is adjusted to maintain a constant proportionality in relation to the level of scintillation light. More specifically, a light-emitting diode is fixed to the glass tube of the PMT. A standard light beam from the light-emitting diode is transmitted through the glass tube, introduced into the light receiving face of PMT, the and converted a into standard level signal. A gain correcting value for the preamplifier is obtained from this standard signal thereby to correct the variation in the output signal. The level of the output signal is thus adjusted to maintain a constant proportionality in relation to the standard light.
However, conventionally the light-emitting diode is located in the resin layer between the glass tube and the circuit housing section of the PMT. The light-emitting diode is thus covered by a cylindrical member made of opaque resin and then embedded in the resin layer. This makes it impossible for the standard light beam to be transmitted through the opaque cylindrical member. In other words, the opaque cylindrical member is low in light transmission efficiency and therefore it is difficult to supply the standard light beam having a sufficient quantity of light to the light receiving face of the PMT. Further, happen that the light-emitting diodes are attached to the glass tube at different angles. As this angle becomes larger, therefore, the transmission of the standard light beam is increasingly disturbed by the opaque cylindrical member. As a result, the intensity of the standard light beam is reduced on the light receiving face of the PMT. Because the standard light beam is not consistently supplied to the light receiving face of the PMT, as described above, the gain of the preamplifier cannot be accurately adjusted.